Permanent ship mooring system

ABSTRACT

A MOORING OR ANCHORING SYSTEM FOR PERMANENTLY LOCATING A SHIP IN A SUBSTANTIALLY FIXED POSITION IN DEEP WATER IS DESCRIBED. A MOORING SWIVEL EXTENDING THROUGH THE HULL OF THE SHIP, NEAR THE BOW, IS MOORED TO THE BOTTOM BY A PLURALITY OF RADIALLY EXTENDING ANCHOR CHAINS. INTERMEDIATE BETWEEN THE ENDS OF THE ANCHOR CHAINS AND THE MOORING SWIVEL, A HEAVY WEIGHT IS PROVIDED ON EACH CHAIN SO THAT THE SHIP IS HELD TO LIMITED EXCURSIONS IN NORMAL SEAS BUT IS PERMITTED GREATER EXCURSIONS IN HEAVY SEAS. A WINCH AND CHAIN STOPS ARE PROVIDED FOR PRE-TENSIONING THE ANCHOR CHAINS AND FIXING THEM RELATIVE TO THE MOORING SWIVEL.

Nov. 16, 1971 a. R. NACZKOWSKI 3,620,181

PERMANENT SHIP MOORING SYSTEM Filed July 2, 1969 5 Sheets-Sheet 1 n 23 INVliN'I'OR.

J26 BRUNO I?v NACZKOWSK/ AT TURNE Y Nov 1 1971 a. R. NACZKOWSKI 3.

PERMANENT SHIP MOORING SYSTEM Filed July 2, 1961 5 Sheets-Sheet a DISPLACEMENT B. R. NACZKOWSKI PERMANENT SHIP MOORING SYSTEM Nov. 16, 1971 5 Sheets-Sheet 5 Filed July 3. 1969 FIG.4

IIE

INVIfN'I'UR. BRUNO R. NACZKOWSK/ A T TOR/V5 Y NOV. 1 1971 B. R. NACZKOWSKI PERMANENT SHIP MOORING SYSTEM 5 Sheets-Sheet 4 Filed July 2, 1969 INVIiNTU/L anu/vo R. NACZKOWSK/ BY xQlx J A T TORNE Y Nov. 16, 1971 a. R. NACZKOWSKI 3,620,131

PERMANENT SHIP MOORING SYSTEM Filed July 2, 1969 5 Sheets-Sheet 5 AU -M7 lNVI-IN'I'OR. BRUNO R. NACZKOWSK/ ATTORNEY 3,620,181 PERMANENT SHIP MOORING SYSTEM Bruno R. Naczkowski, Newport Beach, Calif., assignor to North American Rockwell Corporation Filed July 2, 1969, Ser. No. 838,435 Int. Cl. B63b 35/00, 21/24 US. Cl. 114--.5 D 4 Claims ABSTRACT OF THE DISCLOSURE A mooring or anchoring system for permanently locating a ship in a substantially fixed position in deep water is described. A mooring swivel extending through the hull of the ship, near the bow, is moored to the bottom by a plurality of radially extending anchor chains. Intermediate between the ends of the anchor chains and the mooring swivel, a heavy weight is provided on each chain so that the ship is held to limited excursions in normal seas but is permitted greater excursions in heavy seas. A winch and chain stops are provided for pre-tensioning the anchor chains and fixing them relative to the mooring swivel.

BACKGROUND In the offshore production of oil and other subaqueous minerals there is a practical limitation for bottom mounted towers that extend above the surface. This practical limit is in the order of about 600' feet and economic considerations may limit the utility of towers to even shallower depths. An alternative to fixed towers is to pro vide a portion of a production facility on the sea floor and provide a permanently moored floating facility for the balance. When this is done a vertically extending conduit must be provided between the sea floor and the floating facility. In relatively shallow depths and calm Waters permanent interconnection between a floating facility and the sea floor can be handled with conventional techniques. However, when the depths become great the stresses involved in a permanently moored ship become great and, at any depth, the ability to withstand storm conditions must also be provided.

SUMMARY OF THE INVENTION There is therefore provided in practice of this invention a seagoing ship having a rotatable mooring swivel therethrough near the bow for mooring to the bottom. A plurality of anchors in a radiating pattern serve to hold the mooring swivel in position and permit the ship to weathervane around the swivel. Each anchor chain is provided with a heavy weight intermediate the ends for holding the ship in substantially fixed position during normal seas and permitting greater excursions of the ship during heavy seas.

DRAWINGS Objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 illustrates in perspective a ship constructed according to the principles of this invention;

FIG. 2 is a plan view of an arrangement for mooring the Ship;

FIG. 3 is a side view of the ship and mooring to show its dynamic characteristics;

FIG. 4 illustrates a detail of anchor chains for the ship of FIG. 1;

FIG. 5 is a longitudinal section of a mooring swivel and a portion of the ship;

FIG. 6 comprises a top view of the mooring swivel and a portion of the ship;

FIG. 7 illustrates a detail of a drive for the mooring swivel;

FIG. 8 is a cross-section of an anchor chain stop on the swivel; and

FIG. 9 illustrates another cross-section of the chain stop;

FIG. 10 shows in graphical form the mooring loads on a mooring system.

Throughout the drawings like reference numerals refer to like parts.

DESCRIPTION The general relation of a ship, the mooring swivel, mooring system and a riser conduit incorporating the principles of this invention are illustrated in FIG. 1. As illustrated in this embodiment an ocean (or lake) going tanker 10 is provided with internal storage tanks (not shown) for a large quantity of crude oil or the like. This tanker 10 is in the same general form as ocean going tankers with a pointed bow 11. The ship is, however, essentially a barge since no substantial propulsion is provided on board. Propulsion is not required since the ship is substantially permanently moored at an oil producing location and if moving is necessary tugs are employed for propulsion. The pointed bow does, however, serve to minimize the resistance of the ship to environmental forces such as wind, waves and currents. The ship-shape hull of the tanker provides directional stability and acceptable motions in response to wave action.

Since the ship 10 remains on station in the oil producing field at all times a permanent mooring system is provided which will accommodate high sea stltes. In order to obtain the lowest loading on the mooring system in extreme sea states it is important to orient the ship in the direction of least resistance to the environmental forces such as wind, wave and currents. There is, therefore, provided a mooring swivel 12 for mooring the ship. As hereinafter described in greater detail the mooring swivel 12 is mounted so as to allow the vessel to rotate about the substantially vertical axis of the swivel plug 12; and the mooring swivel extends completely through the ships hull from approximately the deck line through to the hull bottom.

In order to provide the maximum directional stability the mooring swivel 12 is located as far forward in the hull of the ship 10 as is possible to allow the vessel to freely weathervane without providing auxiliary propulsion to orient the ship. For automatic orientation, the distance from the center line of the mooring swivel 12 to the bow 11 of the ship should not exceed 20% of the ships length measured from the bow to the stern. When the mooring swivel is located in the forward 20% of the ship shaped vessel the naturally occurring moments due to wind, waves, and current readily orient the sh p in the direction for minimized forces without any auxiliary propulsion. If the swivel is located aft of about 20% of the ships length auxiliary propulsion may be necessary to properly orient the ship. It will be apparent, of course, that if the mooring swivel is located substantially in the center of the ship that auxiliary propulsion is virtually mandatory since little, if any, natural weathervaning can be expected.

The mooring swivel 12 is moored to the ocean floor 13 by a multiple leg, single point mooring system. In this system a plurality of mooring chains 14 are connected only to the swivel 12 as hereinafter described in greater detail and extend downwardly from the bottom of the mooring swivel and outwardly in a radiating pattern to the ocean floor 13 as may be more clearly seen in the plan view of FIG. 2.

In order to provide fluid communication with an underwater oil production facility a vertically extending riser 17 is connected to underwater oil production facilities 18 shown schematically in FIG. 1. A buoy 19 is connected to the riser 17 at a point substantially below the water surface so that the effect of wave action on the buoy 19 and riser is minimized. The buoy provides a substantial tension force on the riser to support the principal weight thereof. A fluid disconnect 21 is provided above the buoy 19 so that the upper portion of the riser can be disconnected from the buoy if desired.

The upper end of the riser passes through the center of the mooring swivel 12 and is connected to a tower 22 above the deck of the ship. In a preferred embodiment the tower 22 is mounted for relative angular motion to minimize stresses on the riser as is described and claimed in copending US. patent application Ser. No. 838,434, filed July 2, 1969 entitled, Oil Production Vessel by George W. Morgan and Bruno R. Naczkowski, and copending U.S. patent application Ser. No. 838,489, filed July 2, 1969 entitled, Underwater Riser and Ship Connection by George W. Morgan. The contents of said copending US. patent applications are hereby incorporated by reference for full force and effect as if set forth in full herein.

Referring now to FIGS. 2 and 3, the arrangement of anchor chains beneath the ship can be seen in greater detail. In its normal position the ship is stationed with the mooring swivel 12 directly above the oil production facilities 18 and the riser 17 extends directly therebetween. The eight anchor chains 14 extend radially from the mooring swivel as seen in the plan view of FIG. 2 and terminate in anchors 23 which are preferably Navy type stockless or Danforth anchors which lie in a substantially circular pattern on the ocean floor. In a typical situation wherein the ship is moored in about 1500 feet of water the circle of anchors is approximately one mile in diameter.

Intermediate on the anchor chain 14 between the mooring swivel and the anchor on each chain there is provided a lifting clump 24, each of which, for example, may comprise a seventy ton concrete block or other similar material that is considerably heavier than water and which has sufficient strength to retain its shape under a lifting and dropping action such as may prevail under storm conditions. The shape of the clump 24 preferably provides a large landing area to minimize penetration into the ocean bottom upon dropping, and if desired, a conventional landing mat (not shown) can be placed beneath the clump 24 on soft bottoms to preclude sinking of the clump into the bottom and creating excessive suction forces upon lifting. In a typical situation wherein a ship is moored in 1500 feet of water the clumps 24 on the anchor chains are arrayed on a circle approximately one-half mile in diameter.

Between each of the anchors 23 and the clump 24 there is provided approximately one quarter mile of conventional heavy anchor chain 26, which, as seen in the detail view of FIG. 4, terminates in a conventional swivel 27 adjacent the clump 24. A short length of chain 28 extends from the swivel 27 to a second swivel 29 beyond the clump. A shackle 31 on the short length of chain 28 is connected to the lifting clump 24. The second swivel 29 is in turn connected to about 450 feet of conventional anchor chain 32 which is in turn connected by way of a swivel 33 to a heavy duty wire rope 34. Different lengths of chain would be used in other depths. Referring again to FIG. 3 at the upper end of the Wire rope 34 a heavy duty chain 36 is provided for connection to the mooring swivel 12 with a predetermined tension as hereinafter described in greater detail.

There are basically two types of ship mooring systems; the gravity systems which develop horizontal and vertical forces from the lifting of a heavy weight of anchor chain; and the elastic systems wherein a cable or rope is connected to a fixed anchor point at the bottom and the elasticity of the cable rather than its weight, provides restraint.

An elastic mooring system, for practical excursions of the ship, is essentially linear and reacts as a relatively soft system with considerable horizontal displacement of the ship under load of wind, waves, and current. The force restoring the ship to its neutral position is approximately linearly increased as the ship displaces. The gravity systems on the other hand, are non-linear in the practical range, being soft for relatively small excursions and stiff for relatively larger horizontal excursions as more and more chain weight is lifted to provide a restoring force. Gravity systems for mooring can also be limited in useful depth due to the large weight of chain and the limiting strength of the upper segments of the chain.

An ideal system for permanent ship mooring in an oil production field, for example, is a non-linear system which should be stiff for small excursions and soft for large horizontal excursions of the ship. This keeps the ship directly over the riser until a storm develops sufficient force to move the vessel away from the neutral position of the riser pipe (from which it may be disconnected if desired) and then the mooring system is in the soft range whereby the vessel can move freely with the wave motions without substantial increase in mooring chain loads.

This ideal system can be most closely approached by the mooring arrangement hereinabove and illustrated in FIGS. 2 and 3. In this mooring arrangement the upper suspended catenary portion of the mooring system is formed principally of wire rope 34 which has a high strength-to-weight ratio and therefore does not place undue stress on the upper portion of the mooring system. Nearer the bottom a length of chain 32 is provided to resist the abrasion that occurs when the chain is in contact with the ocean floor. The lifting clump 24 concentrates the weight needed in the gravity system at its most effective point to serve as an essentially fixed anchor during normal conditions thereby limiting the scope of motion of the mooring leg between the clump and the ship in the manner of a stiff elastic system. When storm or surge conditions prevail at the surface the forces in the anchoring system increase to the point where the concentrated weight of the clump may be lifted and the scope of motion of the upper portion of the mooring legs increases considerably. This lifting of the clump brings the weight of the chain between the clump and anchor into play and the mooring system is then in the soft range of a gravity system.

Action of the mooring system during a heavy sea state can be seen in the phantom view of FIG. 4. As illustrated therein the ship 10 weathervanes in a direction corresponding to the wind and wave forces generated and may drift off of the position directly above the base of the riser 17 by as much as 8% of the water depth. When in this position tension on the further of the anchor chains 14a lifts the clump 24a off of the bottom and also lifts a substantial portion of the chain 26a off of its normal position on the bottom. Thus the mooring weight applied by the anchor chain 14a is increased by the weight of the clump 24a and chain 26a thereby substantially increasing the force tending to restore the ship to its position directly above the riser. At the same time tension in the opposite anchor chain 14b is relaxed and it sinks under the influence of the weight of the chain 3212. It will be recognized, of course, that only two of the eight mooring chains 14 are illustrated in FIG. 4 and that in a typical dynamic situation two or three of the anchoring clumps may be lifted off the bottom at any time to provide a restoring force to the ship.

The plurality of radially extending mooring chains 14 serves to hold the ship in substantially the center of the circle of anchors at all times. The anchors 23 are at the end of a sufficient chain length that the chain 26 lies along the bottom for an appreciable distance adjacent the anchor and therefore the anchors have minor lifting forces thereon and are fully capable of resisting the horizontal forces and remaining fixed in position under any conditions.

The fixed position of the anchors in a radial pattern also serves to maintain the clumps 24 in a substantially circular pattern on the ocean floor. Because of the constraining force of the radial pattern of mooring chains 26 none of the clumps can displace from their intended radial position. Even when a clump may be lifted when the ship drifts in a direction different from the radius to that clump, only a very small change in clump position can occur which introduces a negligible change in the neutral position of the ship at the water surface.

FIG. illustrates, in a graphical format, the loads on the mooring system as a function of displacement of the ship from a position directly over the riser base. When the ship is moored in a position directly over the riser base the displacement is Zero and a mooring load corresponding to point A on the graph is present in the mooring lines due to pretension applied to the anchor chains. As the ship displaces from a position directly over the riser base under the action of wind, waves and current, the mooring load increases along the line ABC; that is, the force exerted by the ship on the mooring lines and tending to disturb the portion of the anchoring system on the sea floor increases along the nonlinear curve ABC.

'If the mooring system simply had anchors in or on the sea floor as previously provided, the mooring load exerted by the ship would continue toincrease along the line BCD and it can be seen that once the slack is taken up in the pretensioned mooring system by steady state displacement or offset, the mooring load increases at an exceptionally high rate with very small additional displacement from the central position over the riser base. The surge load due to the ship traveling down a wave slope in a storm can cause significant displacement and increased mooring load. In a simple anchor system as hereinbefore provided, this vastly increased mooring lo-ad would drag the anchors and the ship would no longer be in position over the riser base when the displacing force was removed.

In a system as herein described, having a lifting clump between the anchor and the moored ship, the mooring load increases along the curve ABC as hereinabove described, in the steady state condition. The horizontal distance or dispacement to point C is the maximum permissible offset of the ship from the riser base in steady state conditions. If the sea conditions become severe enough to increase the displacement of the ship by a distance greater than the maximum offset to point C, one or more of the lifting clumps is raised from the sea floor thereby bringing a greater length of mooring chain into play and significantly decreasing the rate of increase of mooring load with increasing displacement. Once a clump has lifted from the sea floor the mooring load with increasing displacement proceeds along the curve BCE so that the mooring system can accommodate a much larger displacement without substantially increasing the mooring load.

Such a situation can arise, for example, in a storm or other heavy seas wherein wind, waves and current have displaced the ship to aproximately its maximum offset such as at point C. Thereafter as the ship surges and heaves due to wave action a substantially increased transient and cyclic displacement occurs and an anchor clump is lifted from the sea floor, thereby accommodating the displacement without increasing the mooring load to the point that the anchors would be disturbed. Thus as the ship is displaced from its position directly over the riser base the tension in the mooring legs increases from point A along the curve ABC. When the load reaches the point C one or more of the clumps lifts from the sea floor and the mooring load then increases along the curve CEF.

It will be apparent that the weight of the anchor clump and the size and length of the chain between the anchor clump and the anchor can readily be adjusted for any given water depth to provide a selected mooring load as a function of displacement so that the mooring load does not proceed up the curve CEF to a point suflicient to disturb the anchors or break the anchor chains. The total expected displacement of a ship from the position directly over the riser base can be determined since the steady state loads tending to displace a ship to approximately point C can be found for any desired location. The magnitude of the surge displacement in a storm of any given magnitude can also be estimated; thus for example, in a storm having foot waves a large ship may surge a distance of about 30 feet as the waves pass along its length. After the force tending to displace the ship beyond point C is relaxed, the clump again sinks toward the bottom and the load on the mooring system drops back along the curve FEGB with the clump landing on the bottom again at about pont G. Thus, in a heavy storm the ship may surge substantial distances beyond its maximum steady state offset and the mooring load follows, or stays within, the curve BCEGB as a function of that displacement.

Thus the mooring system resists the relatively steady state forces of wind, waves and current and maintains the ship within a maximum offset from a position directly over the riser base and in case of heavy storm conditions, the displacement of the ship from over the riser base is permitted to increase without substantial increase in the mooring loads. The surge displacement of the ship due to the thrust of wave slope is oscillatory in nature about the offset point of the ship and the mooring system has the strength to withstand the steady state mooring load and yet has adequate flexibility to allow the surge oscillations during a storm without applying an inordinate amount of surge force on the mooring system.

FIGS. 5 and 6 show the arrangement of the mooring swivel 12 in the bow of the ship 10. The tower 22 is deleted from these figures for clarity. As illustrated in this embodiment there is provided a cylindrical water tight well 37 extending through the hull from the bottom up to near the deck line. The mooring swivel 12 within the well 37 comprises an outer cylindrical shell 38 and a peripheral flange portion 39 at the upper end. A plurality of heavy duty wheels 41 beneath the flange 39 transmit the weight of the mooring swivel and the vertical mooring loads from the swivel to the ship. In addition, a plurality of wheels 42 on vertical axes transmit transverse loads between the upper portion of the mooring swivel and the ship.

Stiffening ribs 43 within the shell 38 of the mooring swivel not only stiffen the shell but also provide support for a plurality of ha wse tubes 44 which extend from the top of the mooring swivel to the bottom. Eight hawse tubes are provided in this embodiment, one for each of the anchor chains and the hawse tubes are gently curving from top to bottom to gradually change the direction of each mooring chain as it extends through the mooring swivel. At the lower end of the mooring swivel there is provided a box-like flange 46 extending both inwardly and outwardly from the shell 38. The outer portion of the flange 46 bears against the well 37 in the ship in a bearing surface permanently lubricated with coatings of Teflon (polytetrafluoroethylene) or the like. If desired, a fresh water flush 45 can be provided at the underwater bearing between the flange 46 and the well 37 to inhibit the growth of marine organisms. The inner portion of the flange 46 provides a structural ring stiffening the bottom of the shell 38 and prevents damage to the hawse tubes due to impact with the riser 17 when the ship drifts a substantial distance ofl the vertical position over the riser or rolls or pitches severely.

The bearings at the top and bottom of the mooring swivel 12 permit it to rotate relative to the ship (or the ship to rotate relative to the fixed mooring swivel) in response to weathervaning forces on the ship. It will be recognized, of course, that a very substantial friction may exist between the mooring swivel and ship because of the high loads imposed by the mooring system on the swivel. In order to overcome this friction there may be provided a drive for assisting relative rotation of the swivel and ship. Such a drive is conveniently provided by a rack 47 on the periphery of the flange 39. As illustrated in greater detail in FIG. 7 a hydraulic or electric motor 49 mounted on a movable base on the ship drives a pinion 51 which may engage the rack 47. A hydraulic actuator 52 positions the motor and pinion into or out of engagement with the rack as desired. Under many conditions the ship may freely turn about the swivel with no additional driving force, however, it may be desirable in some instances, such as, for example, in initial mooring of the ship, to deliberately position the mooring swivel by means of the motor 49. Thus the actuator 52 is provided for selectively engaging or disengaging the pinion and rack. It might be noted that any motion between the ship and swivel is relatively slow and normally 30 rotation would require about minutes.

Referring again to FIG. 5 the portion of chain 36 at the upper end of each of the anchor chains 14 passes through one of the hawse tubes 44 and over a conventional chain idler 53 in a chain stop 54 described in greater detail hereinafter. After passing over the chain idler 53 the chain 36 is wrapped around a conventional chain windlass 56 for tightening and then conveyed to a chain locker 57 near the bow of the ship. Chain idlers 58 are provided on the deck of the ship for convenience in initial mooring as hereinafter described. A conventional crane boom 59 may also be provided on the deck for handling the heavy chain.

In the course of mooring a ship as hereinabove described the anchors 23 and clumps 24 are placed on the bottom in their desired position by suitably maneuvering the ship or by carrying the anchors and clumps with an auxiliary vessel. Upon placing the anchors and clumps the upper portions of the mooring chains 14 above the clumps are substantially slack so that the ship can be maneuvered. After placing the anchors and clumps the chain 36 in each of the eight legs of the anchoring system is drawn in by way of the Windlass 56 through its respective chain stop 54 until a selected tension is obtained in each of the anchor chains. The reacting force during tensioning is the opposite chain which is anchored to the bottom. If desired, a second windlass and chain locker (not shown) may be employed during tensioning for pulling on the opposite one of the eight anchor chains to provide a react ing force. By pulling on two chains simultaneously some time may be saved in centering the ship over the riser base since it is desirable to have all chains in substantially equal tension in the neutral position. A sufiicient tension is introduced into each of the anchor chains 14 to hold the ship to a selected excursion from its central position without lifting the clumps from the bottom. Normally this permitted lateral excursion would be less than about 5% of the water depth.

The eight anchor chains are individually tensioned by maneuvering the ship around the mooring swivel and taking in the anchor chains 36 to a selected tension load. After tensioning each anchor chain with the Windlass as illustrated in FIGS. 5 and 6 (or a pair of chains if desired) the chain is locked by its respective chain stop 54 as described in greater detail hereinafter and a conventional removable link is detached between the chain stop and the Windlass so that the anchor chain is free from the Windlass. If no detachable link between shots of chain happens to be adjacent the chain stop, a link is simply cut from the chain with a torch. The ship is then rotated about the mooring swivel until another of the anchor chains is in position to be pretensioned by the Windlass. If desired, the chain can be warped around chain idlers 58 8 between chain stops 54 and the Windlass 56 for pretensioning some of the anchor chains without aligning the chain stop and Windlass in the position illustrated in FIGS. 5 and 6. It will also be apparent that conventional pelican hooks (not shown) and the like may be employed as desired for handling the heavy anchor chain.

After the anchor chains have been pretensioned they are each looked in position at a respective chain stop 54 and the excess chain beyond the chain stop is removed by a detachable link or by cutting the link in the chain. This assures that no excess of chain remains on deck and clear access to the tower 22 above the mooring swivel is assured during operation. The use of several chain stops also permits use of a single pair of windlasses for mooring and minimizes the equipment mounted on the swivel. Tension is maintained in each of the anchor chains by means of a chain stop 54, one of which is illustrated in greater detail in FIGS. 8 and 9.

As illustrated in this embodiment the chain stop which is mounted on the flange 39 of the swivel plug at the end of each hawse tube 44 comprises a housing 61 in which the chain idler 53 is mounted. A chain stopper lever 62 is mounted in the housing for pivotal motion about an axis parallel to the axis of the chain idler 53. A heavy tang or pawl 63 on the chain stopper lever 62 bears against a link 65 of the chain which, in turn, is pressed against a wide bearing post 64 within the housing and fitted within the groove of the chain idler 53. Gravity causes the chain stopper lever 62 to act in the manner of a ratchet as the chain is taken on board the ship; that is, as the chain passes up through the hawse tube 44 toward the Windlass, alternate links of the chain force the pawl 63 on the lever into the position shown in phantom in FIG. 8. If desired spring biasing a handle 66 on the chain stopper lever passes through an opening 67 in the housing 61 and limits motion of the lever. Gravity, however, causes the tang 63 to fall back between alternate links of the chain and the chain is thereby prevented from falling back through the hawse tube since every other link may be engaged by jamming between the stopper lever 62 and the post 64. A hook 68 is pivotally mounted on the housing 61 with an opening engaging the handle 66 so that the stopper lever 62 can be locked in position with the tang against the chain to prevent accidental disengagement.

As an added safety feature, after the chain is locked in position by the chain stopper lever, a chain keeper 69 is dropped into position. The chain keeper 69 is mounted for pivotal motion about an axis parallel to the axis of the chain idler 53 and can be raised as shown in phantom to permit free passage of the chain. When it is desired to lock the chain in position, the chain keeper is lowered and a pair of depending legs 71 on the chain keeper straddle a link 72 of the chain 36 to block passage of the next link 73 and lock the chain in position. After locking the chain in position as shown, the tension applied by the Windlass is relaxed and excess chain removed.

Obviously many modifications and variations within the principles of this invention can be employed by one skilled in the art. Thus for example, a plurality of smaller weights can be provided along the chain between the concentrated weight 24 and the anchor 23 in each leg of the anchoring system. Likewise such a mooring system can be employed for other ships than the oil production tanker illustrated, and variations can be made in various details of the chain handling mechanisms and of the mooring swivel.

What is claimed is:

1. A ship mooring system comprising:

a plurality of anchoring members individually connected to a ship, said anchoring members extending downwardly from the ship and substantially radially outwardly from a point below the ship, each of said anchoring members comprising:

a marine anchor at the remote end of the anchoring member;

a heavy concentrated weight intermediate between the ends of the anchoring member;

relatively heavy anchor chain between the concentrated weight and the anchor;

10 a pivotally mounted pawl for engaging a link on said chain and cooperating with said post for limiting motion of said chain to one direction. 4. An improved anchoring member comprising seriaa relatively light weight tension carrying cable between 5 tirn:

romprising:

a mooring swivel mounted in the ship for rotation relative thereto about a substantially vertical axis, said mooring swivel being located no further aft from the bow of the ship than about 20% of the bow to stern length of the ship, and extending through the bottom of the ship, said mooring swivel including a plurality of guide means, one of said terminal lengths of chain extending through each of said guide means;

means on the ship for applying a predetermined tension to each of said anchoring members; and

a plurality of chain stop means on said mooring swivel for locking each of the terminal lengths of chain in position for maintaining tension in the anchoring member when disconnected from said means for applying a tension thereto.

3. A ship mooring system as defined in claim 2 wherein each of said chain stop means comprises:

ahousing;

a chain idler in said housing for directing chain between said guide means and said means for applying tension;

a bearing post in said housing for engaging chain on said idler;

a marine anchor;

a substantial length of heavy anchor chain connected to said anchor;

a massive weight connected to the heavy anchor chain;

a short length of anchor chain connected to the weight for resisting bottom abrasion;

a substantial length of high strength-to-weight ratio flexible cable connected to the short length of anchor chain for carrying tension;

a short length of anchor chain connected to the cable for connection with conventional chain handling anchor mechanisms on board a ship.

References Cited UNITED STATES PATENTS 3,103,200 9/1963 Fulkerson et a1 ll4-23O 3,111,926 11/1963 Shatto 114206 3,191,201 6/1965 Richardson et al. 114-0.5 D X 3,279,404 10/ 1966 Richardson 114-05 D FOREIGN PATENTS 146,774 7/1920 Great Britain 1l4230 TRYGVE M. BLIX, Primary Examiner U.S. Cl. X.R. 

